1. Field of the Invention
The invention relates to the technology of regenerating a particulate trap used to remove particulates from the exhaust gases of an automotive internal combustion engine and, more particularly, to an ignition device which more economically ignites the particulates to initiate regeneration. This application is an improvement related to U.S. Pat. No. 4,544,388, by the same inventors, directed to apparatus that permits electrically energized regeneration.
2. Description of the Prior Art
Electrical heating elements have been devised to ignite particulate collections in porous traps or filters for internal combustion engines, particularly for diesel engines. Particulates, in case of diesel exhaust emissions, is a term used herein to describe carbonaceous solids and condensible matter as defined by the U.S. Environmental Protection Agency. To date such heating elements have been either (a) embedded at or near the front face of the particulate trap (filters having a high trapping efficiency in the range of 50-90%) to ignite the dense particulate collection for removal by oxidation (see U.S. Pat. Nos. 4,516,993 and 4,523,935), or (b) have been embedded in a support element (an element providing little or no trapping efficiency) up stream from the particulate trap to heat the gaseous flow to an adequate temperature which, in turn, ignites the front part of the particulate collection in the trap (see U.S. Pat. Nos. 4,544,388 and 4,427,418).
In either case the ignition temperature required is relatively high demanding that the power wattage be at a level of at least 1500 watts or more to raise state-of-the-art resistance elements to above that temperature. This results from two factors. First, the electrical resistance means only indirectly heats the particulates because the gaseous flow passing therethrough is heated directly and, in turn, heats the particulates. Particulates must be heated to a level of at least a 1000.degree. F. in order to ignite unless subjected to a catalyst which lowers the ignition temperature to the range of 800.degree. F. Secondly, the electrical heating means heats the entire cross-sectional area of the entrance of the particulate trap which is a very extensive area requiring greater heat content. Thirdly, the particulate collection, as contained in a wall-flow particulate filter, exposes only the edges of the particulate columns to the frontal flow which reduces the effectiveness of heated gaseous flow to achieve ignition. [A wall-flow particulate trap has columns which present cells to the front face of the flow, the cells being relatively few per square inch across such face; alternate columns or cells are closed forcing the flow to penetrate laterally or sideways through the wall before being permitted to exit in an alternate cell or channel.]
What is needed is an electrical ignition device which requires considerably less energy to ignite the particulate collection. Such device should provide for heating directly a small siphoned quantity of the particulates independent from the primary dence collection of particulates; such siphoned quantity is non-layered so that it can be easily heated by conduction from electrical wires adjacent to the particulates. Such device needs to be exposed to only a small portion of the area of the flow, such as 20% or less, to be effective.